The present invention relates to a method and a device for reliably monitoring a closed position of two parts which can move relative to one another. More particularly, the invention relates to a protective door switch and method for monitoring a closed position of a protective door installed for safeguarding a technical installation operating in an automated fashion.
Prior art devices of this type are primarily used in the field of machine and installation safety when the goal is to avoid hazards arising from an installation which operates in an automated fashion. In addition to other protective measures such as light barriers, safety mats, two-hand switches etc., protective doors or flaps or the like are frequently used to separate persons from hazardous locations. The doors are typically provided with an actuator-sensor combination for detecting an opening of the protective door in a reliable, i.e. failsafe, manner so that the machine or installation can be placed in a non-dangerous state, if appropriate. Suitable actuator-sensor combinations thus belong to the field of components which typically require an approval from competent supervisory authorities before use. The present invention relates in particular to devices and methods of the type mentioned at the beginning which meet at least category 3, preferably even the highest category 4, of pertinent European standard EN 954-1 or comparable safety requirements for safeguarding technical installations.
Among the actuator-sensor combinations available in the art, those which interact in a non-contact fashion have been given particular attention. Such devices are generally based on magnetic and/or electromagnetic couplings between the actuator and the sensor, which couplings are interrupted or disrupted when the protective door opens. However, magnetic sensors cannot be used, or can only be used to some degree, in some fields, such as in the metal cutting machining of metallic workpieces.
In order to achieve the required failsafety, prior art devices are generally designed with two-channel redundancy. By way of example, reference is made to documents DE 40 41 550 A1 or DE 44 38 039 A1.
Document EP 0 968 567 B1 discloses a prior art device where the actuator is a transponder which can transmit an internally stored code to a sensor (referred to as a reading head in that document) in a non-contact fashion if the distance between the actuator and sensor falls short of a specific switching distance. An evaluation device which has two-channel redundancy is connected downstream of the sensor-end receiver circuit which is used to read out the transponder, said evaluation device evaluating the sensor signal supplied by the receiver circuit and opening, as a function thereof, two outputend relays with which, for example, a monitored machine is then switched off.
This known device has the disadvantage that when a plurality of protective doors are monitored a corresponding number of evaluation devices is necessary. As an alternative to this, it is basically conceivable to evaluate a plurality of actuators with a common evaluation device; however, in this case either the reaction time of the device and/or its complexity increases.